I used to have problems playing video clips on my computer. Every so often I’d get a video file in a format that wasn’t recognized, or the audio track would not sync up with the video. I’d try it with different players: Quicktime, Real, Windows Media, VLC—all to no avail. It turned out to be a problem with codecs. If I didn’t have the right codec installed, it didn’t matter which player I used. (“Codecs?” you ask, “What are codecs?” Essentially they are plugins for media players that allow them to understand various formats. More info here.) Enter the K-Lite Codec Pack, which contains every known codec. Since I’ve installed K-Lite, I haven’t had any problems playing various media formats.
Note that the “official” site requires you to register with a pay service, but the codec pack is available for free at the link above.